Compound semiconductor monolithic microwave integrated circuits (MMICs) have been widely used for the RF transmitter, receiver, and transceiver in microwave communication devices such as cell phones and wireless LAN modules. The RF modules are composed of many circuit elements such as power amplifiers (PAs), switches, filters, and control devices. Some of those circuit elements are integrated in one chip. A compound semiconductor amplifier (HBT or HEMT) often uses circuits using HEMTs for controlling the transistor bias condition. Those circuits can be integrated in one compound semiconductor chip. For example, the integration of HBTs and HEMTs is achieved by using a BiFET (or BiHEMT) process, in which both HBT PAs and HEMT control circuits are fabricated in a compound semiconductor chip. Another example is to use a process in which enhancement-mode HEMTs and depletion-mode HEMTs are integrated. The enhancement-mode HEMTs are used for a PAs and the depletion-mode HEMTs are used for the control circuits. A compound semiconductor amplifier (HBT or HEMT) and a switch circuit that changes the RF signal path depending on the output power level, frequency band, and the communication mode are also often integrated in one chip. A compound semiconductor amplifier (HBT or HEMT) and an antenna switch circuit that switches the connection of the antenna to different Tx and Rx circuits are also often integrated in one chip. The compound semiconductor HBT PA is often operated at different bias conditions for the different output powers and frequencies to maintain an optimal performance. Since the input and output impedances are functions of the bias condition, an impedance tuner is introduced to maintain a good impedance matching in accordance with the change in the bias condition. The impedance tuner usually consists of capacitors, inductors and HEMT switches. The HEMT switch is used to change the connection of the capacitor and inductor to change the overall impedance. The high integration of circuit elements induces high process cost and low process yield. That is particularly the case when both HBT and HEMT are integrated on one chip.
To reduce the process cost, the circuit elements of the RF module described above can be formed on separate chips, and other electronic chips such as Si CMOS chip can be included. Conventionally, the chips are placed in one plane. However, the use of multiple chips in one plane makes the module size large, and the long interconnection between those chips induces signal loss and interference. An example of such an RF module is the one consisting of an HBT PA MMIC chip, impedance matching and bias control chip, an antenna switch chip, and a filter circuit chip, all of which are placed in one plane on the module substrate.